Our results in the preceding funding period lead us to propose that immunodominance in CD4 T cell responses is largely dictated by an intrinsic property of the peptide:class II complex, which is measured by its kinetic stability. This single parameter can be rationally and experimentally manipulated to either promote or eliminate immune responses. Mechanistically, we have determined that DM editing in APC is a key event that is altered by the kinetic stability of class II:peptide complexes. Accordingly high stability complexes will be expressed at the cell surface of the priming APC at much greater initial densities than other, competing low stability peptides. In this renewal application, using the tools and knowledge of peptide:MHC class II interactions that we have developed, we will shift our attention from the role of antigen processing in determining immunodominance to the dynamic and kinetic aspects of APC-T cell interactions in establishing peptide hierarchies in CD4 T cell responses. We will critically test whether the initial epitope density is the sole element that programs immunodominance hierarchies. We hypothesize that the kinetic stability of peptide:class II complexes may play additional roles in CD4 T cell priming, independent of DM effects and that T cell hierarchies may be re-shaped over time and by ongoing simultaneous responses to different peptides within the protein antigen. Below are our experimental plans to address these issues. Specific Aim 1: Determine when and through what mechanisms immunodominance hierarchies in CD4 T cell responses are initiated and maintained Specific Aim 2. Determine the impact of simultaneous, competing CD4 T cell responses to unrelated peptides in shaping immunodominance. These will provide a new and significant view of the kinetics of acquisition and maintenance of the antigen-specific repertoire and competitive nature of T cell activation, expansion and contraction that are regulated by T cell receptor engagement. The insight gained from these clarify the studies will help provide insight into the factors that shape the specificity in CD4 T cell responses to pathogens and will have significant impact in the design of vaccines that seek to either focus or alternatively to diversify the specificity of CD4 T cell responses. Public Health Relevance: In the previous funding period, we discovered a single critical factor that determines what segments in the pathogen or vaccines are selected be the focus of the immune response. In the experiments planned, we will determine how this single variable focuses the immune response towards such limited regions of the e pathogen. The results of these experiments will help in the design of vaccines that focus or diversify the immune response to pathogenic organisms.